It’s hard enough when your favorite team’s best player leaves town. It’s even harder to watch that star player join a rival.

And nothing in fandom compares to the pain of watching that star take the team you hate to heights your favorite club never reached.

When a star jumps from one rival to another, it doesn’t just leave fans heartbroken and replica jerseys unwearable. And it doesn’t just alter the trajectories of the teams involved. Those rivalry switches can change an entire league.

In the NFL, New York Giants fans have felt the sting of Saquon Barkley’s intra-division relocation with each Philadelphia Eagles win. Less than a year after leaving one NFC East team for another, Barkley has put together one of the best seasons for a running back in NFL history, and he has Philadelphia one win away from a championship.

But Giants fans, you’re not alone.

You can always call up fans of the Oklahoma City Thunder. Or the Boston Red Sox. Or Arsenal supporters across the Atlantic.

Rivals acquiring each other’s key players have shaped the outcome for generations of fans. Here’s how some of those acquisitions went down, and how their ripple effects shaped entire leagues.

NFL: How the Giants let Barkley slip away

One to two million dollars. That was the gap the Giants and Barkley couldn’t bridge after eight months of negotiations despite both sides insisting they desired to reach a long-term agreement.

Barkley rejected a three-year offer believed to be worth $37.5 million during the Giants’ bye week midway through the 2022 season. That set the stage for the protracted, failed negotiations during the 2023 offseason.

The Giants reportedly increased their offer to $13 million per year early in the 2023 offseason, but Barkley again declined because the deal only included $19.5 million guaranteed. A team source said the Giants’ strongest offer before the franchise tag deadline included $23.5 million guaranteed. At that point, Barkley set his sights on Christian McCaffrey’s market-setting contract that carried a $16 million annual salary.

Giants general manager Joe Schoen was in a sticky spot because quarterback Daniel Jones, who had just delivered a virtuoso performance in the franchise’s first playoff win in 11 years, was also a free agent. In Schoen’s ideal world, he would have extended Barkley early so he’d have the franchise tag as leverage in negotiations with Jones.

At a stalemate with Barkley, Schoen shifted his focus to Jones. The sides agreed to a four-year, $160 million extension minutes before the franchise tag deadline in March 2023. The Giants immediately slapped the tag on Barkley, locking in both players for at least one more season.

The final attempts to reach a deal with Barkley came in July 2023. When they failed to agree on a long-term contract, Barkley was required to play on the one-year franchise tag for $10.1 million.

A half-hearted holdout threat landed Barkley a revised deal with $909,000 in incentives before the start of training camp that was supposed to be a sign of goodwill. He didn’t come close to earning the incentives that were based on lofty personal production and team success.

The expectation was that the sides would engage in another round of contentious negotiations during the offseason. But then the 2023 season concluded and there was no extension offer from the Giants before free agency. The Giants planned to let Barkley test the market and consider matching his best offer if it fit within their budget.

The reality was, once they told Barkley to test the market, he was gone. Especially when the Pennsylvania native received a lucrative offer from the Eagles.

Things couldn’t have worked out better for Barkley, who became the ninth player in NFL history to rush for 2,000 yards in a regular season, before advancing to the Super Bowl after enduring years of losing in New York. Meanwhile, losing Barkley contributed to a disastrous season for the Giants, who tied for the worst record in the league at 3-14.

That the Giants’ offseason saga was documented on “Hard Knocks” only added to the scrutiny. Co-owner John Mara’s angst about losing the face of the franchise was captured by his comment to Schoen, “I’m going to have a tough time sleeping if Saquon goes to Philadelphia.”

There have been a lot of sleepless nights in New York as Barkley enjoys a dream season in Philadelphia. — Dan Duggan, Giants beat writer

MLB: Who else but Babe Ruth? There are a few …

The New York Yankees have won a few championships with star players who came directly from Boston, like Johnny Damon, Wade Boggs and, of course, Babe Ruth. But all of them took four years to win a title with the Yankees — and the Yankees had never won a pennant before acquiring Ruth, so there wasn’t much of a rivalry with the Red Sox at the time.

Red Sox fans show their displeasure with Johnny Damon after the centerfielder left Boston for New York. (Jim McIsaac / Getty Images)

Reggie Jackson went from the Baltimore Orioles to the Yankees before the 1977 season, and though the franchises weren’t front-line rivals, they did battle for division supremacy.

Roger Clemens made his name with the Red Sox and nearly pitched them to a title in 1986. He left Boston for a two-year, two-Cy Young interlude with Toronto, then joined a Bronx dynasty in progress. Clemens didn’t elevate the Yankees the way Barkley has the Eagles, though the pitcher helped keep the Yankees on top, with championships in 1999 and 2000. Those Yankees teams remain the last to win consecutive World Series.

A more direct comparison to Barkley — though somewhat more obscure — might be Bruce Sutter, the Hall of Fame closer who was traded from the Chicago Cubs to the St. Louis Cardinals before the 1981 season. Sutter, who won a Cy Young Award with the hapless Cubs, immediately thrived with the rival Cardinals, making the All-Star team in 1981 and closing out Game 7 of the World Series the next October.

But for long-term impact on what would become a fierce rivalry, nothing compares to The Babe.

The Red Sox dominated the first two decades of the American League, winning six pennants and five World Series. And had the New York (Baseball) Giants actually contested the 1904 Fall Classic — rather than decline to play — the Red Sox might have gone six-for-six.

By contrast, the inept New York Highlanders weren’t yet known as the Yankees, and they weren’t yet known for donning iconic pinstripes, or for fielding winning teams. But they were known for employing a star first baseman named Hal Chase, who gained a rep for throwing games in exchange for cash considerations.

Then the Red Sox sold Ruth to the Yankees in 1920. Four years later, the Yankees won the first of the franchise’s 27 world championships, while the Red Sox spent decades getting their hearts broken and lamenting a curse. — Tyler Kepner, national MLB writer

NBA: Kevin Durant loses to the Warriors — then joins them

Before stars joined forces in Oakland, the ones above had to align.

Kevin Durant may have ended up with the Warriors in 2016, helping Golden State cement a dynasty, but first, every step had to go right.

Had Klay Thompson not caught fire during Game 6 of the 2016 Western Conference Finals, which the Warriors once trailed 3-1 and eventually won in seven games, then Durant’s Thunder likely go to the NBA Finals — and Durant doesn’t end up with the Warriors a month later.

Had the Warriors not let go of a 3-1 series lead themselves in the next round, relinquishing the advantage to LeBron James’ Cleveland Cavaliers, then Durant signs somewhere else in free agency. The Warriors set a regular-season record with 73 wins in 2015-16. But they lost their last game.

Durant’s arrival was to ensure that would not happen again.

Thunder fans let Kevin Durant know how they felt when he returned to Oklahoma City for the first time as a Warriors player. (J Pat Carter / Getty Images)

Had the NBA not opened up an unprecedented financial moment, then Durant would not have even had the option to sign with Golden State. An influx of new money stemming from the league’s fresh national television deal was set to kick in for the 2016-17 season, spiking the salary cap from $70 million to $94 million. Never before had the cap jumped so much from season to season — and it hasn’t happened since. Without that eccentricity, Golden State wouldn’t have had the space to sign Durant, who carried them to the next two titles.

Had Stephen Curry not had ankle issues early in his career, then the Warriors wouldn’t have had the space, either. Curry, a two-time MVP at this point, was still on a team-friendly four-year contract, which he agreed to when his health was still in question. If he were making the max, the Warriors couldn’t have signed Durant.

The Warriors already won a championship in 2015 and came one victory away from another the following spring. They won two more in 2017 and 2018 and could have won an extra with Durant in 2019 before he and Thompson both got hurt in the finals.

They created one of the league’s greatest teams because every ounce of fortune, even the losses, went in their favor. And they got a couple of rings in the process. — Fred Katz, national NBA writer

WNBA: Sylvia Fowles joins a Minnesota Lynx dynasty

The WNBA’s three-decade history has been rife with stars joining forces and powerhouse teams assembling and fading. Like Candace Parker, who signed with her hometown Chicago Sky in 2021 and helped them win a title, starring as a secondary scoring threat. Or Breanna Stewart joining the New York Liberty in 2023, forming a superteam and getting the Liberty over the title hump in her second season.

But Sylvia Fowles’ impact on the Minnesota Lynx in 2015 was tough to top.

After seven dominant years as a perennial MVP candidate with the Sky, including leading Chicago to a WNBA Finals appearance in 2014, Fowles entered 2015 requesting a trade. She refused to sign a new deal and sat out the first half of the 2015 season as she looked to be dealt to one specific team.

That deal finally materialized in late July, as Chicago sent her to the Lynx in a three-team deal. In Minnesota, she teamed up with reigning MVP Maya Moore and future Hall of Famers Seimone Augustus and Lindsay Whalen, who had already led Minnesota on title runs in 2011 and 2013.

Sylvia Fowles won two championships after a trade to the Minnesota Lynx. (Hannah Foslien / Getty Images)

Fowles injected her All-Defensive prowess immediately upon arrival, averaging 15.3 points, 8.3 rebounds and 1.6 blocks per game in the regular season. She saved her finest works for the finals, leading Minnesota to a championship with three 20-point performances in the series, including a 20-point, 11-rebound double-double in the clinching Game 5.

Much like Durant in Golden State, Fowles’ acquisition not only provided star-power punch, but also extended the franchise’s title window, turning a powerhouse team into a dynastic one.

Fowles won Defensive Player of the Year in 2016 as Minnesota made another finals run, then took home MVP in 2017 as she helped Minnesota to its fourth championship in seven years.

Soccer: Robin Van Persie trades Arsenal red for Manchester United red

When it comes to soccer, especially in recent years, has there been a more transformational signing than Manchester United buying Robin van Persie from Arsenal in August 2012?

Van Persie had been at Arsenal since the 2004-05 season, but had little to no success in terms of silverware. Yes, he won the FA Cup at the end of his first year in North London, but no other major honors followed.

Having scored 132 goals in 278 games for Arsenal during a time when United and Chelsea were dominating the Premier League, he found it too difficult to resist when United manager Sir Alex Ferguson came calling.

For a meager sum, at least in today’s world of transfers, of £24 million ($29.2 million at today’s rate), Van Persie altered how he is remembered in English football. The Dutch striker scored 26 top-flight goals and was arguably the club’s most important player, helping United win the Premier League in what turned out to be Ferguson’s final year in charge.

Robin Van Persie and Sir Alex Ferguson pose after Van Persie joined from Arsenal. (Andrew Yates / AFP via Getty Images)

Older examples include Rio Ferdinand, who was widely regarded as one of the best central defenders in England at the time, leaving Leeds United as a 23-year-old to join United in July 2002 for around £30 million ($37.2 million at today’s rate), where he went on to win his maiden league title in his first season at Old Trafford.

Another defensive transfer that proved to be transformative for the player and club was Sol Campbell, who left Tottenham Hotspur to join Arsenal. Campbell played for nine years in Spurs’ first team and had one League Cup triumph to his name.

In the five seasons he spent at Arsenal — Spurs’ fierce rivals — he won two FA Cups and two Premier Leagues, including the 2003-04 season when he was an integral part of the side that went through the whole top-flight campaign without losing a match on their way to the title. — Dan Sheldon, Manchester United correspondent

F1: Lewis Hamilton’s move to Ferrari … to be determined

The full effects of Lewis Hamilton’s shock move from Mercedes to Ferrari in 2025 are still to be felt, but it certainly has the potential to change Formula One history.

Hamilton wrote his F1 legacy as a Mercedes driver. Since joining the team in 2013 from McLaren, who gave him a debut in 2007 and a first world championship win the following year, he’d enjoyed unparalleled levels of success. Six world championships in seven years made Hamilton F1’s statistical greatest of all time, tying Michael Schumacher’s record of seven titles (albeit with more race wins).

Hamilton always intended to see out his career with Mercedes, saying in 2023 he wanted to be with the team “until the end of my days.” A new contract was agreed that summer, including an option that would take him to the end of 2025.

Ferrari’s fans have already welcomed in Lewis Hamilton, but the effects of his move are to be determined. (Rudy Carezzevoli / Getty Images)

But before the 2024 season had even started, Hamilton announced he would be leaving Mercedes to join Ferrari, once the bitter rival he fought against for world championships. The appeal of driving the famous red cars was too great for Hamilton to resist. He claimed racing for Ferrari had been a childhood dream, to the extent he’d even raced as Schumacher when playing F1 video games as a teenager.

It is without a doubt the biggest and most shocking driver move in F1 history, and one that will define the final chapter of Hamilton’s career. Off the back of three difficult seasons with Mercedes, where the team struggled with its car and, through 2024, Hamilton trailed his teammate, Ferrari gives the chance for a fresh start. And fresh hope of a record eighth world championship, one he came within a lap of winning in 2021.

The marketing impact of such a megastar partnership as Hamilton and Ferrari is enormous for F1. The first images of him at Ferrari’s factory quickly went viral, and Hamilton made an effort to endear himself to the tifosi, its fervent, loyal fans, who watched his first test on a cold January day. Once their rival, he is their new hero.

Whether it is a success or a failure, Hamilton’s “last dance” with Ferrari is going to be a defining moment in F1’s history — and compelling to watch unfold. — Luke Smith, F1 writer

(Illustration: Will Tullos / The Athletic. Photos: Bettmann Archive, Mitchell Leff / Getty Images)